The present invention relates to computer-implemented trapping processes.
As the use of desktop publishing increases, techniques for preparing and reproducing pages on a variety of output devices have been developed. Such output devices include image setters, printers, monitors, and digital printing presses.
A page may include various types of objects, such as text, line art, and images, and its appearance is generally described by page description language ("PDL") instructions that are interpreted by an output device to create physical output having the desired appearance.
A color page additionally includes information of the colors used in the page. Colors are defined in accordance with a "color space," which provides a data representation of a range of colors (a "color gamut") in terms of basic color components. The specific color components will vary depending on the color system used. For example, in the CMYK color system, colors are represented as combinations of cyan (C), magenta (M), yellow (Y), and black (or "key") (K); in the RGB color system, colors are represented as combinations of red (R), green (G), and blue (B).
A color in the page is generally represented by a series of bits, with specific bits indicating the amount of each basic color component used in the color. For example, a 24-bit RGB data representation may allocate bits 0-7 to indicate the amount of red, bits 8-15 to indicate the amount of green, and bits 16-23 to indicate the amount of blue.
These data representations of colors are used to create color separations, which are used by color output devices to output the page. Generally, each color separation used by an output device will correspond to a color component of the color system used by the device. For example, data representations of colors in a page output on an image setter using a CMYK color system will be used to create color separations for cyan, magenta, yellow, and black, with each separation indicating regions where the corresponding ink should be applied, as well as the amount of ink that should be applied. The same color data for the page, if output on an image setter using an RGB color system, will be used to create color separations for red, green, and blue inks. The output page includes the combination of each of the inks, applied according to its corresponding separation.
Misalignment of separations can cause unintentional gaps or overlaps at edges of color regions in the output page. For example, at an edge separating two regions of different colors, misalignment may result in an output page having a gap between the two regions. At an edge separating a color region from a paper (no color) region, if the color is created from multiple separations, misalignment of those separations may result in an output page having one separation ink extending beyond the other separation inks for that color region.
To minimize the effects of misalignment, a technique known as trapping adjusts the shapes of color regions at these edges by spreading (expanding) some separation color regions to prevent gaps, and choking (contracting) other separation color regions to prevent overlaps. The adjusted areas into which inks will be spread or from which inks will be contracted are referred to as "trap regions," and trapping also entails determination of the amount of ink to be applied to the trap regions for each affected separation.
Trapping thus involves adjusting the separations created by an output device for a page. Because separations correspond to the inks used by an output device, and because different output devices may use different inks, trapping has traditionally been considered to be device-dependent.
Retrapping is used for output devices using inks of different color systems (e.g., RGB inks versus CMYK inks), as well as for output devices using the same color system. For example, two devices may both use a CMYK system, but the inks of one device may be different shades from the inks of the other device. Accordingly, a page having an output color created from, for example, just the magenta ink on a first output device, may be created from a combination of magenta and cyan inks on a second output devices, and therefore, may be trapped differently on the two devices.
Another reason why retrapping may be required is that two output devices may have different color gamuts--i.e., a specific color provided on a first output device may not be within the range of colors available on a second output device. In this case, the second device will approximate the specified color with the closest available color, but the ink combination used will likely be different from that used by the first device, and therefore may require separate trapping.
Yet another source of differences is that colors of a page are often represented in a color space (the "page color space") independent of any device color space. At output, each device uses separations created by converting the page colors into its specific device color space, and any differences in the conversion for different output devices may result in different separations requiring different trapping.
For reasons such as these, a color page is generally not trapped until an output device is specified, and is retrapped for each output device using different separations.